KR20110075627A - Api steel plate for line pipe and method for manufacturing the api steel plate - Google Patents

Abstract

PURPOSE: An API steel sheet for a line pipe and a manufacturing method thereof are provided to enable application of the API steel sheet to an oil pipeline installed in a cold region. CONSTITUTION: A method for manufacturing an API steel sheet for a line pipe is as follows. A steel slab is heated to a temperature of 1210-1280°C and extracted at a temperature of 1140-1160°C. Rough rolling is implemented in a condition where the reduction ratio per pass of the final three passes is 6.0-7.0% below the austenite recrystallization temperature(Tnr). Finish rolling is implemented at a rolling initiation temperature of 915-945°C. Cooling implemented to lower the temperature of the steel sheet from 710-770°C to 450-500°C at a cooling rate of 10-60°C/sec.

Description

API Steel Plate for Line Pipe and Method for Manufacturing the API Steel Plate}

The present invention relates to a steel sheet for a steel pipe that can be used as a line pipe in a sub-zero environment and a sub-zero environment, and more particularly, to excellent low-temperature toughness and to coarse during DWTT (Drop Weight Tear Test) tests. The present invention relates to an API steel sheet for a line pipe which does not cause reverse fracture, and a method of manufacturing the same.

API steels are steels with specifications defined by the American Petroleum Institute, which are used for casing & tubing, which carries oil or gas from deep oil fields to ground, depending on the purpose, and for marine or marine development It is divided into steel used for off-shore structure, and line pipe that carries crude oil or natural gas from oilfield to refinery or refinery to real customer.

Oil pipelines used for petroleum and natural gas transportation are installed from hundreds to thousands of kilometers by welding. Recently, due to the increased cold field and H ₂ S-containing gas field, the installation environment is severe and high strength, low temperature toughness and hydrogen organic The level of demand for cracking properties is increasing.

In particular, in recent years, the reverse rupture generated during the DWTT (Drop Weight Tear Test) test, which is a major evaluation method for low temperature toughness, is also required as one of regulatory requirements.

The DWTT test is a method for evaluating low temperature toughness, in which the area ratio occupied by the toughness portion of the fractured surface fractured by cooling the notched specimen and impacting the opposite side of the notched region with a hammer (i.e. SA).

The reverse fracture refers to the fracture formed on the opposite side of the notched portion during the DWTT test. When the SA is high, the reverse fracture becomes fine.

Techniques for improving the low temperature toughness characteristics of the steel sheet include a method of refining crystal grains by adding at least one of sintering elements such as niobium (Nb), titanium (Ti), and vanadium (V).

However, this conventional method has a limitation in improving coarse reverse breaking generated in the DWTT test.

Therefore, there is a demand for a manufacturing technology of an API steel sheet for a line pipe that is excellent in low temperature toughness and does not cause coarse reverse breakage during DWTT testing.

The present invention provides an API steel sheet for a line pipe and a method of manufacturing the same, which are excellent in low temperature toughness and do not cause coarse reverse breakage during DWTT testing.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is in weight%, carbon (C): 0.04-0.08%, silicon (Si): 0.1-0.4%, manganese (Mn): 1.2-1.7%, niobium (Nb): 0.03-0.06%, aluminum (Al) ): 0.01 to 0.1%, phosphorus (P): 0.020% or less, sulfur (S): 0.004% or less, chromium (Cr): 0.01 to 1.0%, molybdenum (Mo): 0.10 to 0.18%, vanadium (V): 0.03 to 0.06%, balance Fe and other unavoidable impurities;

The tissue consists of acicular ferrite single phase or acicular ferrite of 90 vol.% Or more and bainite of 10 vol.% Or less; And it relates to an API steel sheet for line pipe having a ductility of 90% or more.

One or more selected from the group consisting of nickel (Ni): 0.3% or less, titanium (Ti): 0.03% or less and copper (Cu): 0.01 to 0.3% may be further added to the steel sheet.

In addition, the present invention by weight, carbon (C): 0.04 ~ 0.08%, silicon (Si): 0.1 ~ 0.4%, manganese (Mn): 1.2 ~ 1.7%, niobium (Nb): 0.03 ~ 0.06%, Aluminum (Al): 0.01 to 0.1%, phosphorus (P): 0.020% or less, sulfur (S): 0.004% or less, chromium (Cr): 0.01 to 1.0%, molybdenum (Mo): 0.10 to 0.18%, vanadium ( V): A steel slab containing 0.03 to 0.06%, balance Fe and other unavoidable impurities is heated to 1210 to 1280 ° C, and the slab is extracted to a temperature of 1140 to 1160 ° C, below the austenite recrystallization temperature (Tnr). Rough rolling was performed under the condition that the rolling reduction per pass of the last three passes was 6.0 to 7.0%, followed by finishing rolling under the rolling start temperature condition of 915 to 945 ° C, and starting cooling at 710 to 770 ° C to 10 to 60 degrees. The present invention relates to a method for producing an API steel sheet for a line pipe cooled to a temperature of 450 to 500 ° C. at a cooling rate of ° C./sec.

One or more selected from the group consisting of nickel (Ni): 0.3% or less, titanium (Ti): 0.03% or less and copper (Cu): 0.01 to 0.3% may be further added to the steel slab.

As described above, the steel sheet produced by the present invention is not only excellent in low temperature toughness but coarse reverse fracture does not occur during the DWTT test, it can be applied to a steel pipe for an oil pipe installed in a cold region or the like.

EMBODIMENT OF THE INVENTION Hereinafter, the reason for limitation of the steel composition of this invention is demonstrated in detail.

C: 0.04-0.08 wt%

C is an essential element added to secure the strength of the high-strength pipe, and should be added at least 0.04% by weight to secure sufficient strength.

However, when the C content is more than 0.08% by weight, the cornerstone cementite structure is formed and cracks occur, so that the fatigue strength is significantly lowered. Therefore, the C content is preferably limited to 0.04 to 0.08% by weight.

Si: 0.1-0.4 wt%

Si plays a role of deoxidizing molten steel by assisting Al and has a solid solution strengthening effect that is solid-dissolved in ferrite to strengthen the base material strength, and it is preferable to add 0.1 wt% or more to obtain such an effect.

However, when Si is added in excess of 0.4% by weight, a reddish scale due to Si is formed during rolling, resulting in a bad shape of the steel sheet surface and deteriorating in situ weldability and toughness of the weld heat affected zone. Therefore, the content of Si is preferably limited to 0.1 to 0.4% by weight.

Mn: 1.2-1.7 wt%

Mn is an element that is beneficial to secure the strength by improving the hardenability of the steel when present in the steel, it is preferable to add 1.2% by weight or more in order to improve the hardenability of such steel.

However, when the content of Mn exceeds 1.7% by weight, the hardenability is unnecessarily increased to lower the transformation rate of ferrite during rolling and increase the possibility of low temperature structure during welding. Therefore, the content of Mn is preferably limited to 1.2 to 1.7% by weight.

Niobium (Nb): 0.03 to 0.06 wt%

Niobium (Nb) is an element that exists in the form of compounds (NbC, NbN) in steel and raises austenite recrystallization temperature (Tnr) to prevent coarsening of tissues and to refine grains to improve toughness and ductility. In order to obtain 0.03% by weight or more, but more than 0.06% by weight, since the weldability is lowered and the cost rises, the content of niobium (Nb) is preferably limited to 0.03 to 0.06% by weight.

Chromium (Cr): 0.01 to 1.0 wt%

Cr is an element useful for preventing surface decarburization, securing oxidation resistance and quenching resistance. However, when the Cr content is less than 0.01% by weight, it is difficult to secure sufficient oxidation resistance, surface decarburization and quenching effects. In addition, when exceeding 1.0% by weight may lead to a decrease in the deformation resistance, rather it may lead to a decrease in strength. Therefore, the content of Cr is preferably limited to 0.01 to 1.0% by weight.

Molybdenum (Mo): 0.10 to 0.18% by weight,

Mo is an element that improves hardenability by improving strength and increases austenite recrystallization temperature (Tnr), thereby improving toughness. Molybdenum (Mo) must be added at least 0.10% by weight in order to obtain such an effect, but it is 0.18% by weight. If it is added in excess, it is an expensive element, the cost is increased.

Vanadium (V): 0.03 to 0.06 wt%

V is an element that increases the strength and toughness by forming precipitates such as carbides or nitrides to refine grains. To obtain such an additive effect, V is added in an amount of 0.03% by weight or more. Since the amount of coarse carbide which is not dissolved is increased and the precipitation strengthening effect is lowered, the content of V is preferably limited to 0.03 to 0.06% by weight.

Aluminum (Al): 0.01 to 0.1 wt%

Al is a component added for the purpose of deoxidation, but 0.01 wt% should be added for sufficient deoxidation, but if it is added in excess of 0.1 wt%, non-metallic inclusions are formed to reduce toughness, so the Al content is 0.01 to 0.1 wt%. It is desirable to limit to%.

Titanium (Ti), nickel (Ni) and copper (Cu) are elements added as needed.

Titanium (Ti): less than 0.03% by weight

Ti is added for deoxidation and precipitation strengthening, and its content is preferably limited to 0.03% by weight or less.

Nickel (Ni): 0.3 wt% or less

Ni is an element that refines the steel structure to improve strength and toughness, and the content thereof is preferably limited to 0.3 wt% or less.

Copper (Cu): 0.01 to 0.3 wt%

Cu is an element that improves the corrosion resistance, but in order to improve the corrosion resistance, 0.01 wt% or more should be added, but when it is added in excess of 0.3 wt%, the cost is increased.

Phosphorus (P): 0.020 wt% or less

P is inevitably contained in steel production, and it is preferable to control it as low as possible because it causes brittleness as an element easily segregated in the center of slab during solidification.In theory, it is possible to limit the content of P to 0%, but it is inevitable in manufacturing process. It can only be added. Therefore, it is important to manage the upper limit, the upper limit of the content of P is preferably limited to 0.020% by weight.

Sulfur (S): 0.004 wt% or less

S is an element that is inevitably contained in steel production, and because it has a good affinity with Mn, it is present in the form of MnS and is not compressed during rolling, and has a feature of being elongated. In addition, it is preferable to suppress the content as much as possible because it causes red light brittleness. In theory, it is possible to limit the content of S to 0%, but inevitably be added in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the content of S is preferably limited to 0.004% by weight.

The steel sheet of the present invention has a structure consisting of acicular ferrite single phase or acicular ferrite of 90 vol.% Or more and bainite of 10 vol.% Or less, and has a flexible wavefront ratio of 90% or more.

Therefore, the steel sheet of the present invention does not cause coarse reverse breakage during the DWTT test.

Hereinafter, the manufacturing method of the API steel plate for line pipes of this invention is demonstrated.

The steel slab formed as described above is made of a thick plate through heating, rolling, and cooling processes, and this manufacturing process will be described.

Slab heating and extraction temperature: 1210 ~ 1280 ℃ and 1140 ~ 1160 ℃

The heating process of the slab is a process of smoothly performing the subsequent rolling process and heating the steel to sufficiently obtain the properties of the target steel sheet, so the heating process should be performed within an appropriate temperature range according to the purpose.

What is important in the heating process is that not only the precipitated elements inside the steel sheet should be heated uniformly so as to be sufficiently dissolved, but also the maximum protection against excessive coarsening of grains due to too high a heating temperature is important.

If the heating temperature of the steel is less than 1210 ℃, Nb is not reusable in the steel, it is difficult to achieve high strength of the steel sheet, and partial recrystallization occurs to form austenite grains unevenly, which makes it difficult to toughen the 1280. When it exceeds the ℃ austenite grains are too coarse to provide a cause for the increase in the grain size of the steel sheet, as a result of which the strength and toughness of the steel sheet is extremely degraded. Therefore, the slab heating temperature is preferably limited to 1210 ~ 1280 ℃.

The extraction temperature of the slab heated as above is limited to 1140 ~ 1160 ℃.

Rolling condition

The slab extracted at the above temperature is roughly rolled under the austenite recrystallization temperature (Tnr) under the condition that the reduction ratio per pass of the last three passes is 6.0 to 7.0%.

The rough rolling is preferably performed at least five times of multi-stage rolling at an austenite recrystallization temperature (Tnr) or less.

As described above, by rough rolling under austenite recrystallization temperature (Tnr) under the condition that the rolling reduction per pass of the last three passes becomes 6.0 to 7.0%, the grains are stretched, deformed, and refined, and the dislocations due to deformation are developed inside the grains. By providing a nucleation site that can be transformed into acicular ferrite tissue during cooling to improve the toughness by forming a fine acicular ferrite.

If the rolling reduction per pass of the last three passes is less than 6.0%, the grain size is not sufficiently refined. If the reduction rate exceeds 7.0%, the temperature of the steel sheet after rough rolling becomes too low to make the finish rolling difficult. The reduction ratio per pass is preferably limited to 6.0 to 7.0%.

Next, after rough rolling as described above, finish rolling is carried out under a rolling start temperature condition of 915 to 945 ° C., and cooling is started at 710 to 770 ° C. to 450 to 500 ° C. at a cooling rate of 10 to 60 ° C./sec. Cool to temperature.

Cooling rate: 10 ~ 60 ℃ / sec

If the cooling rate is less than 10 ° C / sec polygon ferrite can be generated a large amount can not secure strength and low temperature toughness.

However, when the cooling rate exceeds 60 ℃ / sec is not only easy to control the cooling end temperature, but too low temperature transformation phase, such as martensite is generated too badly affects the low temperature toughness. In addition, when the cooling rate exceeds 60 ℃ / sec due to the excessive amount of cooling water due to the characteristics of the steel sheet to be the object of the present invention, the distortion of the steel sheet occurs, the shape control is poor.

Therefore, the cooling rate is preferably limited to 10 ~ 60 ℃ / sec.

Cooling end temperature: 450 ~ 500 ℃

In order to control the internal structure of the steel sheet, it is necessary to cool it to a temperature at which the effect of the cooling rate is sufficiently manifested. If the cooling stop temperature, which is the temperature at which the cooling stops, exceeds 500 ° C., it becomes difficult to sufficiently form needle-like ferrite and bainite having fine grains in the steel sheet, and thus the effect of improving the tensile strength is insufficient. Therefore, the upper limit of the said cooling stop temperature needs to be limited to 500 degreeC. However, when the cooling stop temperature is less than 450 ℃ may not only saturate the effect but also martensite is formed due to excessive cooling, which may cause a problem of low temperature toughness.

Therefore, the cooling end temperature is preferably limited to 450 ~ 500 ℃.

The structure of the steel sheet prepared as described above is preferably composed of acicular ferrite single phase or acicular ferrite of 90 vol.% Or more and bainite of 10 vol.% Or less.

In addition, the steel sheet has a ductile fracture rate of 90% or more, and coarse reverse fracture does not occur during the DWTT (Drop Weight Tear Test) test.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

Steel slabs having a thickness of 300 mm satisfying the component system shown in Table 1 were heated to 1220 ° C. to extract the slabs under the extraction temperature conditions of Table 2 below, and roughly rolled under the austenite recrystallization temperature (Tnr) under the conditions of Table 2 below. After the finish rolling and cooling to prepare a steel sheet having a thickness of 24mm, the ductile wave ratio (SA) was measured by the DWTT test, the results are shown in Table 2 together.

Here, the rough rolling was carried out a total of 13 passes, the reduction ratio per pass of the last three passes are shown in Table 2 below.

In addition, the invention material (1) and the comparative material of the following Table 2 observed the occurrence of reverse break during the DWTT test, the results of the invention material 1 is shown in Figure 1, the comparative material is shown in Figure 2. In addition, the microstructure photograph was observed about the invention material (1) and the comparative material, and the result of invention material 1 is shown in FIG. 3, and the comparative material is shown in FIG.

Steel grade
Chemical composition (% by weight) C Si Mn Nb Cr V Ni Mo Ti P * S * Tnr AR3 Invention steel 0.06 0.25 1.55 0.045 0.1 0.05 0.2 0.15 0.015 120 30 993 766 Comparative steel 0.065 0.2 1.55 0.04 0.1 - 0.15 - 0.012 120 20 1000 767

(However, the content unit of the * indicated element in Table 1 is ppm)

Psalm No.
Steel grade Slab extraction temperature (℃) Rolling rate per pass of Jojoshima last three passes (%) Uncrystallized rolling reduction rate at finish rolling (%) Rolling end temperature (℃) Cooling end temperature (℃) Cooling rate
(℃ / S) Ductility Rate (SA) (%) Invention 1
Invention steel
1145 6.4 78 830 475 42 98 Invention 2 1150 6.1 78 830 470 45 94 Invention 3 1155 6.7 78 830 480 50 98 Comparative material Comparative steel 1160 5.6 78 825 480 40 56

As shown in Table 2, the inventive material (1-3) that satisfies the steel composition and manufacturing conditions of the present invention has a SA of 94% or more, excellent toughness, as shown in FIG. , DWTT also occurred finely during DWTT. In addition, in the invention material (1), the structure was needle-like ferrite as shown in FIG.

On the contrary, in the case of the comparative material, the ductile fracture ratio (SA) was 56%, not only good toughness, but also coarse fracture occurred in the DWTT test as shown in FIG. 2.

In the case of the comparative material, the structure was bainitic ferrite as shown in FIG. 4.

1 is a photograph showing a reverse cross section generated during the DWTT test of the invention (1)

Figure 2 is a photograph showing the reverse cross-section generated during the DWTT test of the comparative material

3 is a microstructure photograph of the invention material (1)

4 is a microstructure photograph of the comparative material

Claims (4)

By weight%, Carbon (C): 0.04 to 0.08%, Silicon (Si): 0.1 to 0.4%, Manganese (Mn): 1.2 to 1.7%, Niobium (Nb): 0.03 to 0.06%, Aluminum (Al): 0.01 To 0.1%, phosphorus (P): 0.020% or less, sulfur (S): 0.004% or less, chromium (Cr): 0.01 to 1.0%, molybdenum (Mo): 0.10 to 0.18%, vanadium (V): 0.03 to 0.06 %, Balance Fe and other unavoidable impurities; The tissue consists of acicular ferrite single phase or acicular ferrite of 90 vol.% Or more and bainite of 10 vol.% Or less; And API steel sheets for line pipes having a ductile fracture rate of 90% or more. The steel sheet further comprises one or two or more selected from the group consisting of nickel (Ni): 0.3% or less, titanium (Ti): 0.03% or less and copper (Cu): 0.01 to 0.3%. API steel sheet for line pipe, characterized in that it comprises. By weight%, Carbon (C): 0.04 to 0.08%, Silicon (Si): 0.1 to 0.4%, Manganese (Mn): 1.2 to 1.7%, Niobium (Nb): 0.03 to 0.06%, Aluminum (Al): 0.01 To 0.1%, phosphorus (P): 0.020% or less, sulfur (S): 0.004% or less, chromium (Cr): 0.01 to 1.0%, molybdenum (Mo): 0.10 to 0.18%, vanadium (V): 0.03 to 0.06 Steel slab containing%, remainder Fe and other unavoidable impurities is heated to 1210 ~ 1280 ℃ and the slab is extracted to a temperature of 1140 ~ 1160 ℃, per the last 3 passes under the austenite recrystallization temperature (Tnr) After rough rolling under the condition that the reduction ratio is 6.0 to 7.0%, finish rolling is carried out under the rolling start temperature conditions of 915 to 945 ° C, and the cooling is started at 710 to 770 ° C to cool 10 to 60 ° C / sec. Method for producing an API steel sheet for line pipe cooling to a temperature of 450 ~ 500 ℃ at a speed. The method of claim 3, wherein the steel slab is one or more selected from the group consisting of nickel (Ni): 0.3% or less, titanium (Ti): 0.03% or less and copper (Cu): 0.01 to 0.3%. Method for producing an API steel sheet for a line pipe, characterized in that it comprises a.

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